Re: [Boost-users] [pre-processor ] help needed

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-01-28 13:12:16

AMDG

Ovanes Markarian wrote:
> Hello *,
>
>
> I have some pre-processor sequence in form of:
>
> (X1) (X2) (X3) (X4)
>
> which is expanded by BOOST_PP_SEQ_FOR_EACH to template parameters:
>
> some_template<X1, X2, X3, X4> ...
>
>
> My problem now is that some of Xn parameters can be a template 
> instantiation and that actually causes PP to think, that there are 
> more parameters to the function applied by ..._SEQ_FOR_EACH.
> Example: some_other_template<T1, T2> cause preprocessor to think that 
> there are 2 params some_other_template<T1 and T2>.
>
> Putting the parameter into additional braces like ((X4)) results in 
> the template:
>
> some_template<X1, X2, X3, (X4)> and that results in a compiler error. 
> I know this construct can switch off ADL, but I don't know why it 
> result here in a compiler error, since I fully qualify the X4 (from 
> which namespace it is). Is there any way to treat Xn as a single 
> paramer but during the final pre-processing stage get rid of 
> additional braces?
>

There are a couple of ways to do it.  You can make each element of the 
Seq to be itself a seq
and then call BOOST_PP_SEQ_ENUM on the inside:

((X1))  ((X2))  ((some_other_template<T1)(T2>))

Another way is to use template metaprogramming

(X1)(X2)(parenthesize_type(wrap<some_other_template<T1, T2>>))

template<class T>
struct unparenthesize { typedef T type; };

template<class T>
struct unparenthesize<parenthesized_type(wrap<T>)> { typedef T type; };

In Christ,
Steven Watanabe